Gabor Csathy

Nonconventional Odd-Denominator Fractional Quantum Hall States in the Second Landau Level

We report the observation of a new fractional quantum Hall state in the second Landau level of a two-dimensional electron gas at the Landau level filling factor ν=2+6/13. We find that the model of noninteracting composite fermions can explain the magnitude of gaps of the prominent 2+1/3 and 2+2/3 states. The same model fails, however, to account for the gaps of the 2+2/5 and the newly observed 2+6/13 states suggesting that these two states are of exotic origin.

Observation of a Helical Luttinger-Liquid in InAs/GaSb Quantum Spin Hall Edges

We report on the observation of a helical Luttinger liquid in the edge of an InAs/GaSb quantum spin Hall insulator, which shows characteristic suppression of conductance at low temperature and low bias voltage. Moreover, the conductance shows power-law behavior as a function of temperature and bias voltage. The results underscore the strong electron-electron interaction effect in transport of InAs/GaSb edge states. Because of the fact that the Fermi velocity of the edge modes is controlled by gates, the Luttinger parameter can be fine tuned. Realization of a tunable Luttinger liquid offers a one-dimensional model system for future studies of predicted correlation effects.

Observation of a transition from a topologically ordered to a spontaneously broken symmetry phase

Hydrostatic pressure is used as a means to tune the two-dimensional electron gas hosted in a GaAs/AlGaAs crystal from a topologically ordered to a spontaneously broken symmetry phase.

Contrasting energy scales of reentrant integer quantum Hall states

We report drastically different onset temperatures of the reentrant integer quantum Hall states in the second and third Landau level. This finding is in quantitative disagreement with the Hartree- Fock theory of the bubble phases which is thought to describe these reentrant states. Our results indicate that the number of electrons per bubble in either the second or the third Landau level is likely different than predicted. theories, however, have their limitations. The Hartree- Fock approach, the only one used for bubble phases both the TLL (25, 29, 30) and the SLL (29), is exact only in the limit of large LL occupation (24, 26), and may there- fore not capture all aspects of bubbles at the lowest LL occupation, i.e. those in the second and third LLs. In ad- dition, the presence of competing nearby fractional quan- tum Hall states in the SLL (17, 18) is likely to enhance fluctuations and may therefore influence electron order- ing. Finally, none of the theoretical techniques include LL mixing, an electron-electron interaction effect known to strongly affect the energy gaps of fractional quantum Hall ground states in the SLL (31). Here we report sharp peaks in the temperature depen- dent longitudinal resistance of the RIQHSs in the TLL which are similar to those of the RIQHSs in the SLL. This shared property highlights the common origin of these RIQHSs. The sharp peaks allowed us to extract the onset temperatures of the RIQHSs in the TLL which enabled a quantitative comparison of the RIQHSs form- ing in the TLL with those in the SLL as well as with the theoretically predicted bubble phases. Our measure- ments of the onset temperatures are at odds with the cohesive energy calculations obtained within the Hartree- Fock approximation and indicate that the assignment of the RIQHSs to the various bubble phases is likely differ- ent than predicted.

Effect of Strain on Stripe Phases in the Quantum Hall Regime

Preferential orientation of the stripe phases in the quantum Hall (QH) regime has remained a puzzle since its discovery. We show experimentally and theoretically that the direction of high and low resistance of the two-dimensional (2D) hole gas in the QH regime can be controlled by an external strain. Depending on the sign of the in-plane shear strain, the Hartree-Fock energy of holes or electrons is minimized when the charge density wave (CDW) is oriented along the [110] or [110] directions. We suggest that shear strains due to internal electric fields in the growth direction are responsible for the observed orientation of CDW in pristine electron and hole samples.

Quantitative analysis of the disorder broadening and the intrinsic gap for the

We report a reliable method to estimate the disorder broadening parameter from the scaling of the gaps of the even and major odd denominator fractional quantum Hall states of the second Landau level. We apply this technique to several samples of vastly different densities and grown in different molecular beam epitaxy chambers. Excellent agreement is found between the estimated intrinsic and numerically obtained energy gaps for the ν = 5/2 fractional quantum Hall state. Furthermore, we quantify the dependence of the intrinsic gap at ν = 5/2 on Landau-level mixing.

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We fabricated a He-3 immersion cell for transport measurements of semiconductor nanostructures at ultra low temperatures and in strong magnetic fields. We have a new scheme of field-independent thermometry based on quartz tuning fork Helium-3 viscometry which monitors the local temperature of the samples environment in real time. The operation and measurement circuitry of the quartz viscometer is described in detail. We provide evidence that the temperature of two-dimensional electron gas confined to a GaAs quantum well follows the temperature of the quartz viscometer down to 4 mK.

=5/2 fractional quantum Hall state

We report an unexpected sharp peak in the temperature dependence of the magnetoresistance of the reentrant integer quantum Hall states in the second Landau level. This peak defines the onset temperature of these states. We find that in different spin branches the onset temperatures of the reentrant states scale with the Coulomb energy. This scaling provides direct evidence that Coulomb interactions play an important role in the formation of these reentrant states evincing their collective nature.

Integrated electronic transport and thermometry at milliKelvin temperatures and in strong magnetic fields

The region of filling factors

Collective Nature of the Reentrant Integer Quantum Hall States in the Second Landau Level

1/3<\nu<2/5